I mean, there's 2 perspective at looking why Extra Classes? 1. In school you can't pass a certain subject, or fail to satisfy a score, or grade of the subject, so that's why students have to take extra class to reach a satisfaction. 2. Some, students excel at a particularly subject, and when that student is studying with a system with one standard for everybody, then that student would see it as too weak for them. That's why they have to take extra classes for their satisfaction for their skills to reach a higher goal then just a common standard of everybody.